This application is based on applications Nos. 99-15925 and 99-36933 filed in the Korean Industrial Property Office on May 3, 1999 and Sep. 1, 1999, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to an electrolyte for a rechargeable lithium battery and, more particularly, to an electrolyte for a rechargeable lithium battery which can provide a rechargeable lithium battery having good safety characteristics.
(b) Description of the Related Art
In recent years, the development of miniaturized portable electronics provokes needs for a rechargeable lithium battery having a high capacity as well as a light weight. The rechargeable lithium battery employs materials into or from where lithium ion can be intercalated or deintercalated for positive and negative active materials. In the rechargeable lithium battery, organic or polymer electrolyte is used.
For develop the rechargeable lithium battery having high capacity, the safety characteristics of the battery is very important. According to repeated charge and discharge cycle, lithium is extracted from a negative electrode. The lithium explosively reacts with an electrolyte, which leads the explosion of the rechargeable lithium battery under the remarkable increased in the temperature and pressure due to the extreme condition such overcharge, feed-through or compression.
Studies on active materials, separators, system or electrolytes are attempted to insure the safety characteristics of the rechargeable lithium battery.
One method to insure the safety characteristics of the rechargeable lithium battery is that a porous polymer film having low melting point is used for a separator. In this case, when internal temperature of the battery is remarkably raised, the porous polymer film is melt, thereby blocking pores of the film and preventing the movement of lithium ions between electrodes. The method can theoretically prevent the increases in current and temperature, but because the polymer film for the separator has relatively higher melting point, the method does not effectively prevent the reaction of lithium and electrolyte.
Another method to insure the safety characteristics is that when pressure is increased by the generation of gas in the battery, shut-down of the battery is induced. However, the method does not effectively prevent the reaction of lithium and electrolyte.
Still another method to insure the safety characteristics uses an electrolyte an organic solvent and lithium salts. The organic solvent includes cyclic carbonate such ethylene carbonate, propylene carbonate, chain carbonate such diethyl carbonate, ether, ester or ketone. However, when the electrolyte is employed in a rechargeable lithium battery having high capacity, the satisfied safety effect is not obtained.
Still another method to insure the safety characteristics, U.S. Pat. No. 5,506,068 discloses 1,3-dioxolane for an electrolyte. The electrolyte is prepared by mixing 1,3-dioxolane, LiAsF6 as a lithium salt and tertiary amine as a polymerization inhibitor (stabilizer). The electrolyte thermally polymerizes at 4.0V and more, and 100xc2x0 C. Accordingly, the electrolyte can prevent the diffusion of temperature. Furthermore, a rechargeable lithium battery with the electrolyte has good battery performance. However, because the electrolyte has electrochemically stability at 4.0V and less, the electrolyte is not applicable to a high-capacity rechargeable lithium battery in which cut-off occurs at 4.2 to 4.3V When the electrolyte is used in the high-capacity rechargeable lithium battery, the capacity is reduced by the reaction of electrolyte.
U.S. Pat. No. 5,830,600 discloses that phosphate-based fire-retardant agent and pyrocarbonate-based solvent as CO2-generating compound are added to an electrolyte to insure cycle and safety characteristics. However, the method should use at least 50 volume percent of phosphate-based solvent such that ion conductivity is decreased.
It is an object of the present invention to provide an electrolyte for a rechargeable lithium battery which can provide a rechargeable lithium battery having good safety and high capacity.
It is another object to provide an electrolyte for a rechargeable lithium battery which can provide a rechargeable lithium battery having good initial charge and discharge cycle life.
These and other objects may be achieved by an electrolyte for a rechargeable lithium battery including a lithium salt, an organic solvent and a thermosetting organic compound.